webadmin: Hide content widget of boot protocol label

Took a lot of space when visible, and when the label grew longer in
foreign languages, this would cause other widgets in the dialog to
rearrange.

Change-Id: I9c2de4ba2ad14235e8e59f13c5ccdf9b106a257c
Bug-Url: https://bugzilla.redhat.com/1051524
Signed-off-by: Lior Vernia <lvernia@redhat.com>